This section is intended to introduce the reader to various aspects of art, which may be associated with exemplary embodiments of the present invention, which is described and/or claimed below. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not necessarily as admissions of prior art.
The production of hydrocarbons, such as oil and gas, has been performed for numerous years. To produce these hydrocarbons, one or more wells of a field are typically drilled into a subsurface location, which is generally referred to as a subterranean formation, basin or reservoir. From the wells, lines or pipelines are utilized to carry the hydrocarbons to a surface facility for processing or from surface facility to other locations. These pipelines are typically formed from pipe segments that are welded together at weld joints to form a continuous flow path for various products. As such, these pipelines provide a fluid transport system for a wide variety of products, such as oil, gas, water, coal slurry, etc.
Generally, pipelines may be affected by various forces that damage or rupture the pipeline. Recently, increased demand for oil and gas has provided a significant incentive to place pipelines in geographic regions with large ground deformations. Placing pipelines in these regions presents engineering challenges in pipeline strength and durability that were not previously appreciated or approached. These large ground deformations may occur in seismic regions, such as around fault lines, or in arctic regions. In these regions, pipelines may be subjected to large upheaval or subsidence ground movements that occur from the ground freezing/thawing and/or large horizontal ground movements that occur from earthquake events. Because of the ground movements, pipelines, which may be above or below ground, are subject to large strains and plastic deformation that may disrupt the flow of fluids. Further, various load conditions, such as force-controlled load conditions, may be applied to the pipeline as internal pressures and external pressures. In particular, if the pipeline is subjected to predominantly force-controlled load conditions, an allowable stress design methodology is utilized to ensure that the level of stress in the pipeline remains below the yield strength of the pipeline material.
In addition, because the pipe segments are welded together, the weld joints between the pipe segments or between the pipe segments and auxiliary components, such as elbows or flanges, may provide weak points that are susceptible to these forces. For instance, a weld joint between two pipe segments may have flaws that weaken the pipeline. If the weld joint has flaws, then the pipeline may fail at the weld joint due to load conditions or ground movement. Accordingly, the weld joints of the pipe segments may be designed to have sufficient strength and fracture toughness to prevent failure of the weld joint under large strains. This may be accomplished by selecting a proper weld and pipeline material and geometry and selecting an appropriate welding technique, inspection acceptance criteria, and geometry.
To make such determinations about welds and materials, objective inputs may be used. For instance, one such input is the measurement of tearing resistance. Tearing resistance represents the strength of the crack tip as function of the crack size. Tearing resistance is typically represented as a curved line, evincing the material strengthening while it tears. Typically, tearing resistance curves have been obtained based on a single fracture parameter such as crack tip opening displacement (CTOD) or J-integral. ANDERSON, T. L., Fracture Mechanics: Fundamentals and Applications, 2d ed., CRC Press, Inc. (1995). These parameters are usually measured using geometry-independent specimens. However, at large-scale yielding, the geometry-independent specimens are not valid and geometry-dependent analysis is preferred.
Attempts to more accurately measure tearing resistance at large scale yielding include applying a multiplying factor to increase the measured tearing resistance. Unfortunately, under large scale plasticity the tearing resistance is a function of the geometry and the multiplying factor is an unknown variable. WANG, Y; LIU, M; HORSLEY D; ZHOU, J; “A Quantitative Approach to Tensile Strain Capacity of Pipelines,” IPC2006-10474 (September 2006). A second approach utilizes non-standard specimens, such as Single Edge Notch Tension (SENT) to estimate tearing resistance at large scale yielding. ØSTBY E.; “Fracture control—Offshore pipelines: New strain-based fracture mechanics equations including the effects of biaxial loading, mismatch and misalignment;” 24th Int. Conference on Offshore Mech.'s and Arctic Eng'g; paper OMAE2005-67518, ASME; Halkidki, Greece (June 2005). However, these specimens tend to overestimate the tearing resistance by a variable factor. Some standard methods for determining tearing resistance are described in ASTM E 1820-06, “Standard Test Method for Measurement of Fracture Toughness,” ASTM Int'l; and BS 7448 (parts 1-4), “Fracture Mechanics Toughness Test,” British Standards Institute.
Accordingly, the need exists for a method and apparatus that may be utilized to measure tearing resistance that includes the effects of the geometry of the member being tested and the effect of plastic strain on the tearing resistance.